The high mortality rate associated with many of human cancers, including breast is due to a propensity for these tumors to spread while the primary tumors are small and undetected. One of the molecules that have been shown to influence metastasis is metastatic tumor antigen 1 (MTA1). The last decade has witnessed one of the most exciting periods in the biology of MTAs family in cancer cells. For example, we and others have established that overexpression of MTA1, a master chromatin modifier, is frequently associated with an aggressive clinical course in human breast cancer and its overexpression leads to hyperplasia and tumor formation in transgenic mice. Despite the high significance of MTA1, the founding family member, in the NuRD complex and its wide spread upregulation in the human cancer, it remains unknown whether MTA1 or its direct targets participate in the DNA damage response pathway and thus, could influence the responsiveness of cancer cells to radiation therapy - a major treatment regimen currently used for cancer treatment. In this context, recently, we attempted to assess the significance of MTA1 in DNA damage response in cancer cells. As described below, our preliminary studies have discovered that MTA1 is a critical modifier for efficient DNA damage response as its depletion hypersensitizes cells to ionizing radiation (IR)-induced DNA damage, and that MTA1 can partially restore a defect in DNA repair p53-null cells. We interpret these findings as suggesting that MTA1 plays an inherent role in efficient DNA damage responsiveness and modulation of cancer cell's radio-sensitivity. These findings offer a unique opportunity to study the mechanistic involvement of MTA1 and/or its direct targets in DNA damage response. This revision will test the working hypothesis is that "MTA1 and/or MTA1-targets contribute to DNA damage response via both p53-dependent and -independent manner." Regulation of MTA1 regulation of DNA damage response is likely to have significant consequences relating to the sensitivity versus resistance of cancer cells. This revision proposal represents a long-term commitment of the PI to define the significance of MTA1 or its effectors in the biology of human cancer and breast cancer in-particular. Here we propose to investigate the role of the MTA1 pathway as a new player to the growing network of DNA damage response mechanisms. This revision application will vigorously pursue the following three focused, achievable Specific Aims within the requested 24-months of funding: (1) The role and significance of MTA1 in DNA damage response and mechanistic basis of MTA1 regulation of p53 levels;(2) The molecular basis and significance of MTA1 upregulation of p21Waf1 in DNA damage response;(3) The role of MTA1 as an upstream determinant of sensitivity of breast cancer cells to DSB inducing therapy in physiologic relevant model systems. An innovative aspect of our revision is the focus on a molecule with an established role in cancer cells and use of physiologic relevant models. The PI and his consultants/collaborators believe that the knowledge gained from this project will form the basis for new mechanistic advances and identify novel pathways, connecting NuRD complexes and its targets with the DNA damage response. Our research is significant in that the knowledge-gap to be filled by this revision research will enhance our understanding of a new critical regulatory pathway with established roles in cancer progression. This work will enhance our ability to design strategies to improve the efficacy of IR therapy for breast cancer patients with high MTA1. As molecule studied here and radiotherapy is not cancer specific, lessons learned from the current study will have a significant impact on other cancers with increased MTA1. PUBLIC HEALTH RELEVANCE: Our research is significant in that the knowledge-gap to be filled by this revision research will enhance our understanding of a new critical regulatory pathway with established roles in cancer progression. In addition, this also will enhance our ability to design strategies to improve the efficacy of radiation therapy for breast cancer patients with high MTA1. As molecule studied here and radiotherapy are not cancer specific, lessons learned from the current study will have a significant impact on other cancers with increased MTA1.